Money question about joint accounts/gift tax

[theintended]

Anyone want to weigh in on this scenario? I think I know the answer, but agreed to do some research for a family member.

Aging mother gives joint access (with rights of survivorship) to a savings account to son so he can help manage her money.

Mother dies.

Son closes the account and sets up a joint account with his sister so he can share the money with her.

Can his sister withdraw her share of the money (more than 100K) without someone needing to pay taxes on that amount?

[theintended]

Thanks, Farmer! You're the best!